Coated proppants are often used in hydraulic well fracturing to increase production rate of the well. Recently, we have discovered that cured, commercially acceptable, coatings can be applied to proppants using the polyurethane reaction products of polyols and isocyanates. The details of these processes are disclosed in our co-pending U.S. patent application Ser. No. 13/099,893 (entitled “Coated and Cured Proppants”); Ser. No. 13/188,530 (entitled “Coated and Cured Proppants”); Ser. No. 13/626,055 (entitled “Coated and Cured Proppants”); Ser. No. 13/224,726 (entitled “Dual Function Proppants”) and Ser. No. 13/13/355,969 (entitled “Manufacture of Polymer Coated Proppants”), the disclosures of which are herein incorporated by reference. Such polyurethane-based proppant coatings are economically and environmentally desirable for a number of reasons, all of which suggest that the development and use of such coating would be highly desirable.
Two other published patent applications discuss the use of isocyanates for proppant coatings. Tanguay et al. 2011/0297383 presents examples of high temperature proppant coatings made of a polycarbodiimide coating on sand. The coating is said to be made from the reaction of a monomeric isocyanate and a polymeric isocyanate. The catalyst is a phosphorous-based catalyst exemplified in example 1 by 3-methyl-1-phenyl-2-phospholene oxide.
Tanguay et al. 2012/0018162 relates to a polyamide imide proppant coating for high temperature applications. The examples have a description of the use of polymeric diphenylmethane diisocyanate, trimellitic anhydride, one of three different types of amines, triethylamine as a catalyst, an adhesion promoter and a wetting agent. The coating/reaction process described lasts about 10 minutes followed by a post-cure heating of 1-3 hours.
The commercial “standard” coatings are typically a form of phenolic thermoset coating. Partially cured phenolic proppants are typically used in low temperature wells (i.e., those having bottom hole temperature of less than about 150° F. (66° C.)) which typically exhibit low crack closure stresses (e.g., 2000-6000 psi). The theory behind their use is that the residual reactivity of the partially cured phenolic coating in the context of heated water found in most wells will permit the coating to soften and flow, thereby allowing the proppants to consolidate and form interparticle bonds during the “shut-in” period. The high temperature of the downhole conditions is supposed to complete the curing reactions in situ in the propped formation. An activator fluid is used to soften the outer surface of these precured proppants in an effort to encourage consolidation and interparticle bonding. The activator itself raises, however, additional issues of compatibility with the fracturing and breaker fluids as well as the possibility of adverse effects on the continued conductivity of the fractured strata.
For high temperature wells, such as those with a bottom hole temperature above about 200° F. (93° C.), precured phenolic coatings are often used. The high crack closure stresses are often above 6,000 psi are used as the main mechanism for holding proppant within the cracked strata.
In practice, however, a variety of factors can adversely affect the performance and usefulness of the precured, phenolic coatings. The most important of these is premature curing of the partially cured phenolic resin in the coating due to exposure to high temperatures before introduction into the fractured strata. Even the elevated, above-ground, temperatures found on loading docks and in shipping containers can be enough to effect curing of the coating long before it is desirable.
Thus, there exists a need in the industry for a proppant coating that can be used in high temperature wells that will form interparticle bond strength at the expected downhole temperature and pressure conditions yet will not be compromised in forming such interparticle bond strength by premature exposure to elevated or high temperatures.